1. Field of the Invention
The invention relates to a neutral control apparatus and method for a vehicle including an automatic transmission, and particularly relates to a neutral control apparatus and method which enable continuation of neutral control of an automatic transmission in a vehicle while maintaining the endurance of a friction engagement device of the automatic transmission.
2. Description of the Related Art
A known neutral control apparatus for an automatic transmission connected to a drive power source via a torque converter includes an engagement pressure regulating device that regulates the load of a friction engagement device of the automatic transmission. When the vehicle is at a standstill, the apparatus performs neutral control by placing the automatic transmission in a neutral state by reducing the load of the friction engagement. As one example of apparatuses of this kind, the neutral control apparatus disclosed in Japanese laid-open patent application No. JP2004-353844 controls the engagement load of a friction engagement device, which is engaged when establishing the first speed at the automatic transmission, according to the torque ratio of the torque converter, and allows a small amount of torque to be transmitted through the friction engagement device during the neutral control so that the load on the drive power source decreases and, as a result, the fuel economy improves while ensuring a good responsiveness of the vehicle when it starts moving. This neutral control apparatus is configured to estimate the temperature of the friction engagement device and cancel the neutral control when the estimated temperature exceeds a reference temperature. This is because if the friction engagement device is kept slipping to transmit a small amount of torque, friction will raise the temperature of the friction engagement device up to a level which may significantly reduce the endurance of the friction engagement device.
Such cancellation of the neutral control, however, reduces the fuel economy. Also, even if the foregoing reference temperature above which the neutral control is cancelled is set to a higher level in order to delay the cancellation of the neutral control and thereby improve the fuel economy, it will result in reduced endurance of the friction engagement.